The "Vanderpump Rules" star spilled a little alleged tea about what he says is the real story about this house of theirs that Ariana wants to unload -- the same one they bought together, and where they lived for a long time before their nasty breakup last year.

TS went onto Nick Viall's podcast this week and told his side of the story -- basically, he claims Ariana is being petty ... insisting he's made a cash offer of $3.1 million to keep it for himself.

Tom explains that they purchased this house for significantly cheaper than what he's offering her right now -- so her turning it down, in his mind, is just her continuing to throw a fit about the affair. TS also claims she's hardly there and that she hasn't paid a single bill in months.

TMZ broke the story ... Ariana sued Tom earlier this month in an effort to force a sale of their home, which they bought together back in 2019 for around $2 million.

But now, Tom's firing back, saying ... she's doing the equivalent of flipping a Monopoly board when somebody's losing. That's exactly how he frames it ... likening her to a little girl pouting.

It's interesting ... Tom agrees with Nick here -- who says he can understand where Ariana is coming from, and why she would just want to sell the place without letting him have it.

With that said ... Tom thinks she's just being spiteful for spite's sake. Clearly, he wants this place on his own -- and his cash offer does sound pretty sweet.

Seems like they're both digging in their heels and gearing up for a big fight over the house.